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The present invention relates generally to satellite systems and, more particularly, to a method and an apparatus for preventing interference from signal transmission due to misaligned ground terminals.
Ground terminals are in widespread use in satellite communications systems today. When ground terminals are installed, they are turned on and acquire satellite signals. For satellites in geosynchronous orbits (geosynchronous satellites), such as the C/Ku/Ka-band satellites being used to provide a variety of services including television distribution and two-way communications networks (e.g., VSAT and Internet connection systems) in common use today, acquiring satellite signals at turn-on is not a difficult problem. Geosynchronous satellites appear stationary relative to a fixed point on earth, and so it is relatively simple to point a ground terminal antenna to acquire a geosynchronous satellite signal. On the other hand, geosynchronous satellites are rather closely spaced from each other because only a limited number of satellites can be placed on the geosynchronous orbit. As a result, even a slight misalignment of the ground terminal antenna by a small angle (e.g., about 1-2 degrees) will affect the communication between the ground terminal and the intended or target geosynchronous satellite. The antenna of a ground terminal may be misaligned due to poor installation or disrupted due to weather or other factors. When the ground terminal with the misaligned antenna transmits signals intended for the target satellite, the signals may interfere with the operation of the geosynchronous satellite(s) adjacent to the target satellite. Locating and fixing offending ground terminals is difficult and time-consuming.
Embodiments of the present invention are directed to a method and an apparatus for preventing interference of geosynchronous satellites due to ground stations with misaligned antennas. Preventing this problem is particularly important for broadband systems because they require orders of magnitude more ground terminals than are currently in use to be successful.
An aspect of the present invention is directed to a two-way satellite ground terminal configured to communicate with a geosynchronous target satellite which transmits a target satellite signal. The ground terminal comprises an antenna which is configured to receive the target satellite signal from the geosynchronous target satellite only if the antenna is oriented toward the geosynchronous target satellite. A transmitter is configured to transmit signals from the ground terminal via the antenna. A signal detector is operatively coupled with the antenna and with the transmitter. The signal detector is configured to enable the transmitter to transmit signals from the ground terminal to the target satellite via the antenna only upon detecting the target satellite signal received by the antenna from the geosynchronous target satellite at a signal level above a preset threshold level.
In some embodiments, the transmitter includes a signal interlock switch which is activated by the signal detector to enable the transmitter to transmit signals from the ground terminal to the target satellite via the antenna only upon detecting the target satellite signal received by the antenna from the geosynchronous target satellite at a signal level above the preset threshold level. The signal interlock switch is coupled with a signal source, is normally in an open position, and moves to a closed position to form a signal path from the signal source through the transmitter to the antenna only upon detecting the target satellite signal received by the antenna from the geosynchronous target satellite at a signal level above the preset threshold level. The signal interlock switch is configured to move from the closed position to the open position after a preset period of time upon detecting loss of the target satellite signal. The signal detector is coupled between the antenna and the signal interlock switch by a feedback loop. The target satellite signal may comprise a signal at a specified frequency, or a signal encoded with a specified code. The ground terminal includes a directional antenna that receives signals from and transmits signals to the geosynchronous target satellite.
Another aspect of the invention is directed to a two-way satellite ground terminal configured to communicate with a geosynchronous target satellite which transmits a target satellite signal. The ground terminal comprises an antenna which is configured to receive the target satellite signal from the geosynchronous target satellite only if the antenna is oriented toward the geosynchronous target satellite. The ground terminal further comprises a signal source, and a transmitter operatively coupled between the signal source and the antenna. The transmitter is configured to transmit signals from the signal source via the antenna to the target satellite only if the antenna receives the target satellite signal from the geosynchronous target satellite at a signal level above a preset threshold level.
In some embodiments, a signal interlock switch is operatively coupled between the transmitter and the signal source. The signal interlock switch is normally in an open position and moves to a closed position to form a signal path from the signal source through the transmitter to the antenna only if the antenna receives the target satellite signal from the geosynchronous target satellite at a signal level above the preset threshold level. The signal interlock switch is configured to move from the closed position to the open position after a preset period of time upon detecting loss of the target satellite signal.
Another aspect of the present invention is directed to a method of preventing interfering signals from being transmitted by a two-way satellite ground terminal to unintended satellites, wherein the ground terminal is configured to communicate with a geosynchronous target satellite which transmits a target satellite signal. The method comprises providing an antenna for the ground terminal. The antenna is configured to receive the target satellite signal from the geosynchronous target satellite only if the antenna is oriented toward the geosynchronous target satellite. Signals are transmitted from the ground terminal to the target satellite via the antenna only upon detecting the target satellite signal received by the antenna from the geosynchronous target at a signal level above a preset threshold level.
In some embodiments, transmitting signals from the ground terminal comprises coupling a transmitter to a signal source by activating a signal interlock switch operatively coupled between the transmitter and the signal source to form a signal path from the signal source through the transmitter to the antenna. The signal interlock switch is deactivated to terminate the signal path from the signal source through the transmitter to the antenna after a preset period of time upon detecting loss of the target satellite signal.
In accordance with another aspect of the invention, a two-way satellite ground terminal configured to communicate with a geosynchronous target satellite comprises a servicing antenna, and an aligning antenna connected with the servicing antenna and being configured to receive a verification signal from a geosynchronous reference satellite only if the aligning antenna is oriented toward the geosynchronous reference satellite with the servicing antenna oriented toward the geosynchronous target satellite. A transmitter is configured to transmit signals from the ground terminal via the servicing antenna. A signal detector is operatively coupled with the aligning antenna and with the transmitter. The signal detector is configured to enable the transmitter to transmit signals from the ground terminal to the target satellite via the servicing antenna only upon detecting the verification signal received by the aligning antenna from the geosynchronous reference satellite at a signal level above a preset threshold level.